1. Field of the Invention
The present invention relates to a photoelectric conversion element that can be used in a facsimile, image scanner, or other similar device.
2. Description of the Related Art
Amorphous materials are often used in thin-formed photoelectric conversion devices used in image-reading apparatus such as facsimiles or image scanners. Photoelectric conversion elements formed from such amorphous materials can be broadly divided between the diode-type and TFT-type (Thin Film Transistor). As photoelectric conversion elements of the diode-type, Schottky-type, pi-type (p-intrinsic), and pin-type (p-intrinsic-n) elements using a primary photocurrent are known, and as photoelectric conversion elements of the TFT-type, nin-type and pip-type elements using a secondary photocurrent are known.
In the types using a primary photocurrent, although the .gamma. characteristic showing output linearity is approximately 1 and the response time is approximately 10 microseconds (.about.10 (.mu. sec)), a great burden is placed on the signal processing circuit in order to obtain a sufficient S/N ratio from the small photocurrent obtained. On the other hand, in types using a secondary photocurrent, although a small S/N ratio can be obtained from the large photocurrent, the .gamma. characteristic is on the order of 0.8 and the response time is on the order of 10 milliseconds.
In order to enable high-speed and high-graduation reading in photoelectric conversion devices of the TFT-type, which use a secondary photocurrent, it is necessary to shorten the life of the transient carrier and decrease parasitic capacity. To achieve these goals, the configuration of the photoelectric conversion element, the material of the photoelectric conversion film, and control over the MIS [Metal Insulator Semiconductor] plane and backchannel plane all become crucial.
Japanese Patent Laid-open No. 232366/88 discloses a shield film having dimensions equal to or slightly greater than the photoreception area. This construction enables a great decrease in the amount of overlap of the upper electrode, which is believed to influence response time, and further controls focus of the electric field, thereby allowing suppression of pinhole defects.
Nevertheless, there remains the problem that in a contact-type photoelectric conversion element in which light is introduced from behind the transparent insulation film, reduction of the size of the shield film results in a drastic increase in the light directly introduced into the photoelectric conversion section, thereby causing a large signal output due to light leakage and an increase in apparent dark current resulting in a reduced light/dark ratio.
In addition, in the amorphous silicon layer used in an a-SiTFT of the prior art, a film having extremely small defect density is formed. Because the TFT is formed simultaneously, this film is necessary to provide a satisfactory switch element.
Nevertheless, when the above-described amorphous silicon film is used as a photoelectric conversion element, as described in, for example, Japanese Patent Laid-open No. 161683/88, there is a problem that arises because the light signal becomes unstable due to the occurrence of transient fluctuation in the electric potential of the light shield film, which becomes a gate electrode. This phenomenon is chiefly caused by channels formed in the MIS interface.
Furthermore, there is distortion in the defect level of the amorphous silicon film of the back channel portion, which is the light-receiving plane, and in the band of the bonding plane of the amorphous silicon layer and the insulating layer that serves as a protective layer. Japanese Patent Laid-open Nos. 278468/91, 278478/91, and 278479/91 disclose that by using a silicon nitride film in which the ratio of nitrogen to silicon (N/Si) is 0.5-0.9 as a protective layer, band distortion can be reduced as compared with constructions that utilize an insulating silicon nitride film of the prior art (having the composition ratio [N/Si]=1.3), thus allowing suppression of the formation of an accumulation layer formed in the back channel portion. This is because the optical gap is largely 5.3 (eV) in the insulating silicon nitride layer and about 2.0 (eV) in the silicon-rich silicon film, and the energy difference as compared with the amorphous silicon film of about 1.73 (eV) is extremely small.
However, if a silicon-rich silicon nitride film is formed on the photoelectric conversion element as a protective layer, the above-described silicon-rich silicon nitride layer has a photoelectric conversion ability for light having a wavelength of 570 nm, and therefore, the light-receiving portion is actually larger, and in theory, this means that despite an increase in photocurrent, response time lengthens because photoelectric charge is held in the vicinity of the element. This phenomenon has been confirmed in experiments conducted by the inventors.
Furthermore, regarding the manufacturing process of the photoelectric conversion element, if a low-resistance film formed to connect the photoelectric conversion film and the upper metal electrode forms on the backchannel portion (light-receiving surface), it must be removed by a dry-etching method, and this etching gives rise to lattice defects due to plasma damage in the photoelectric conversion film directly below the low-resistance film. These lattice defects easily combine with impurities during the manufacturing process, and if these impurities are electrically charged bodies, there is the problem in that interaction with the light carrier and fluctuation due to electric potential gradient between electrodes influence the characteristic of the photoelectric conversion element.
Of the above-described photoelectric conversion elements of the prior art, the element described in Japanese Patent Laid-open No. 232366/88 suffers from the drawback that reduction of shield film size results in a radical increase in light directly incident to the photoelectric converter, and large signal output due to leakage of light and a increase of apparent dark current results in a reduced light/dark ratio.
In addition, there is the problem that in an amorphous silicon film used in an a-SiTFT, the optical signal becomes unstable due to transient fluctuation of the electrical potential in the shield film, which is a gate electrode.
Furthermore, in elements wherein a silicon nitride film having a nitrogen-to-silicon ratio [N/Si] of 0.5-0.9 is employed as a protective layer, as described in Japanese Patent Laid-open Nos. 278468/91, 278478/91, and 278479/91, there is the problem that in forming a silicon-rich silicon nitride film with a photoelectric conversion ability for light having a wavelength of 570 nm as a protective layer on a photoelectric conversion element, the light-receiving portion effectively increases in size, so that although in principle the photocurrent increases, response time is conversely lengthened because a photoelectric charge is held in the vicinity of the element.
Furthermore, there is the problem that manufacturing processes tend to bring about dispersion in the characteristics of photoelectric conversion elements, resulting in instability.